In an electrical-discharge-machining process for machining a workpiece by applying a pulsed voltage between the workpiece and an electrode through a dielectric fluid to generate discharge, it is necessary to control conductivity (specific resistance) of the dielectric fluid to fall within a predetermined range to make it easier to generate discharge under stable conditions. In addition, to prevent the workpiece from being dissolved because of a change in a hydrogen ion concentration (or hydroxyl ion concentration) of the dielectric fluid as a result of control over the conductivity of the dielectric fluid, it is necessary to control a pH of the dielectric fluid to fall within a predetermined range determined according to a quality of a material for the workpiece. There has been conventionally proposed a technique for measuring the conductivity and the pH of the workpiece used in the electrical-discharge-machining process by a conductivity meter and a pH meter, respectively, and for controlling the fluid quality of the dielectric fluid based on measurement results (see, for example, Patent Documents 1 to 3).
Furthermore, in the electrical-discharge-machining process, if the workpiece is immersed into the dielectric fluid for a long time, metal that constitutes the workpiece is corroded. To prevent the metal corrosion, a corrosion inhibitor is sometimes added to the fluid. However, when a concentration of the corrosion inhibitor increases, then the conductivity of the dielectric fluid increases and stable discharge cannot be performed. To prevent this disadvantage, there has been conventionally proposed an iron-based metal corrosion protecting method capable of protecting an iron-based metal in water from corrosion using an anion exchange resin in which one or more types of ions from among nitride ions and carbonate ions, hydrogen carbonate ions, and hydroxyl ions are fixed, and ensuring stable discharge (see, for example, Patent Document 4). In addition, there has been conventionally proposed a corrosion protecting method for protecting a cemented carbide or a metal in water from corrosion by adding a water-soluble metal anticorrosive to the water although the method is not applied to the electrical-discharge-machining process (see, for example, Patent Document 5).
Patent Document 1: Japanese Patent Application Laid-Open No. S63-191514
Patent Document 2: Japanese Patent Application Laid-Open No. H4-141319
Patent Document 3: Japanese Patent Application Laid-Open No. H5-42414
Patent Document 4: Japanese Patent Application Laid-Open No. 2000-301624
Patent Document 5: Japanese Patent Application Laid-Open No. H7-145491